Discovery: MDNS does not find devices across multiple interfaces

[bltavares]

I found that running the discovery on mdns only worked for one interface. As soon as I tried it on a multihomed device, such as my laptop with WiFi and Cable on different networks, I could not find it anymore.

The discovery has been documented on socket-2, which shows that while we can listen on multiple interfaces for the incoming packets, we need to do some gymnastics to send packets on multiple interfaces.

Goal

Create a single socket which listen to multicast packets on multiple interfaces, and can send packets on a multiple interfaces as well.

What we gonna do on the live stream?

The goal is to create a socket interacting with socket-2 that provides an API to multiple interfaces. This will require having Rust interact with C++/C structs and do C->Rust FFI (which I have no idea how to do that much.) Come learn with me.

Create a Multicast UDP socket that:

• Listens for a specific multicast ip:port (eg: MDNS)
• Join multicast group on multiple interfaces
• Receive a message: (data, origin ip, origin interface)
• Send a message: (data, destination ip, origin interface)
• Send a message on all active interfaces: (data, destination ip, [all interfaces])

References

• Support for sending multicast packets on multiple interfaces with a single socket rust-lang/socket2#86
• https://stackoverflow.com/questions/32278414/c-listen-to-multicast-on-all-interfaces-respond-on-same-as-recieved
• https://docs.microsoft.com/en-us/previous-versions/windows/desktop/legacy/ms741687(v=vs.85)
• Incoming interface would be useful for multicast UDP rust-lang/socket2#29
• Add cmsg PKTINFO for IPv4 and IPv6. nix-rust/nix#990

[bltavares]

Now that we have multicast-socket we can plan ahead to use it on our project.

TODO Next stream:

Livestream	Details
Planned date	Sunday 10am GMT-3 - 2020-07-05 in pt-br
Link	https://twitch.tv/bltavares_codes
Timezone	https://everytimezone.com/s/f2f1d2af

• Use Multicast Socket on colmeia-hyperswarm-mdns